The purpose of this project is to determine the distribution of calcium in cellular organelles and the effects of physiologic stimuli on the distribution of calcium and the morphology of calcium-containing organelles. It is particularly concerned with the question whether the endoplasmic reticulum (ER) is a calcium uptake system and whether calcium can be released form the ER in response to surface membrane stimuli such as specific ligand-receptor interactions. Tissue is prepared by rapid freezing or by modified fixation procedures designed to retain Ca in situ. Ca is identified in sections by electron-probe x-ray microanalysis. Early stages of this study have been devoted to refinement and testing of the methods.